The present invention relates to the catalytic hydrodechlorination of chloromethane compounds, e.g., carbon tetrachloride or chlorofluoromethanes, using a novel class of catalyst. The term "chloromethane" as used herein is intended to cover those methane compounds which have one or more of its hydrogen atoms substituted by chlorine, optionally also substituted by fluorine, and is capable of being catalytically hydrodechlorinated when reacted with hydrogen. It should also be understood that the present invention is useful with mixtures of such chloromethane compounds as well as compositions comprising only one such chloromethane compound.
For example, chlorofluorocarbons in the atmosphere are a matter of current concern regarding depletion of the ozone layer. Chlorofluorocarbons containing lower amounts of chlorine have been proposed as a substitute since the carbon-hydrogen bonds in such substitutes are more easily cleaned by chemical reactions at lower altitudes in the atmosphere than carbon-chlorine bonds thereby preventing the compound from reaching higher altitudes where ozone is present (see Inorganic Chemistry, Vol. 31, No. 10, 1992, pp. 1965-1968).
Noble metal catalysts, such as those containing palladium, have been proposed for use in the hydrode-halogenation of halogen-substituted hydrocarbons containing fluorine. Examples of U.S. patents relating to the use of this type of catalyst include: U.S. Pat. Nos. 4,980,324; 5,057,470; 5,094,988; and 5,202,510.
Transition metal carbide catalysts form a distinctly differing class of catalyst and have been proposed for use, in general, in a variety of processes including ammonia decomposition, hydrogenation and dehydrogenation, carbon monoxide methanation, isomerization, oxidation, hydrodesulfurization, and hydrodenitrogenation reactions. (see Catalysis Today, 15, 1992, pp. 179-200).
Tungsten carbide granules, which had been mixed with aluminum hydroxide to form a paste which was then spread onto an alumina honeycomb and heated at 500.degree. C. to produce a catalyst, were also employed in the production of tetrafluoroethane from a haloethane containing four or five fluorine atoms, rather than in the treatment of a chlorofluoromethane compound. (See Japanese Patent Publication No. 01/319,442). Although the stability of saturated fluorocarbons exceeds that of their chlorocarbon analogues, the stability of the ethane-based fluorocarbons is less than that of the methane analogues (see R. E. Banks, Fluorocarbons and Their Derivatives, Macdonald Technical & Scientific, London, 1970, pp. 17-18).
In regard to the catalytic hydrodechlorination of carbon tetrachloride, platinum and palladium catalysts are described for use in U.S. Pat. No. 3,579,596, in the Journal of Catalysis, 22, 245-254 (1971) and in German Offenlegungsschrift No. 4,138,141, for example. Other catalysts, comprising nickel, cobalt and copper on zeolites, have been shown to reduce the chlorine content in carbon tetrachloride, forming chloroform and 1,1,1,2-tetrachloroethane in the Journal of Catalysis, 34, 136-143 (1982).